Modeling the structure of high porosity asteroids

A number of asteroids have been observed to have extremely low bulk densiti es in the 1.3 g/cm(3) range. Given reasonable compositions based on meteori te analogues, this bulk density implies bulk porosities of 40-60%, suggesti ng that many asteroids are rubble piles held together by self-gravity and c ontaining significant empty space, perhaps more empty space than solid mate rial. Simple energy balance calculations and accretional models of disrupte d asteroids suggest that materials in porous asteroids may be sorted by par ticle size. The large irregular pieces land larger voids/fractures) are pro bably located deeper inside the asteroid and the fine particle size fractio ns that are observed on their surfaces are restricted to the surface regoli th zone, The large interior voids/fractures are preserved from infilling by the effects of friction on the smaller size fractions. Friction tends to d ominate the downward pull of gravity and prevents the fine fractions from f iltering into the interior of the asteroid and infilling the large fracture s and voids. Friction may also play a role in allowing shattered asteroids to maintain their relief features and shape by resisting the movements of p ieces within the object, in effect providing strength to non-coherent objec ts. (C) 2001 Academic Press.